In the building construction industry it is common to secure a layer of thermally insulating material to a substrate, for example to a large sheet of corrugated metal. One way in which this has been accomplished is to fasten the layer to the substrate with a plurality of spaced-apart screw fasteners each extending from the outer side of the insulating layer to the opposite side of the substrate. For example, a screw, with a large apertured plate under its head serving a washer-like function, may be inserted through the insulation and screwed into and partially through the underlying substrate. In one known manner of accomplishing this, the screw fastener is of the self-piercing, self-tapping type, so that as it advances it creates its own opening in the substrate and then forms internal threads in the opening with which the external threads of the fastener engage as they are screwed through the substrate.
While adequate for many purposes, the foregoing assembly is subject to the following difficulties, particularly in the case in which the layer is of a resilient material. If the fastener is subjected to substantial and abnormal forces urging it toward the substrate, as when personnel or equipment are present on the exterior of the layer, the screw threads may be subjected to so much force that the screw fastener is forcefully advanced axially through the threaded opening, the mating threads being sufficiently deformed by the large applied force that they can slip over each other to permit such advance of the screw. During such an event, the insulating layer is compressed to a smaller thickness than its normal thickness, and when the source of the unusual force on the fastener is removed, the mating threads engage sufficiently to prevent the return of the layer to its normal thickness, as would otherwise occur. Thus a one-way action is produced whereby once the layer has been compressed in this manner, it will remain in its compressed state thereafter rather than returning to its original, desired normal thickness.
Furthermore, in many such applications this insulating layer and screws made are covered with one or more additional layers of material. For example, it is common to provide a so-called "built-up roof" or membrane over the top of a resilient insulating layer held to a substrate by screws, the membrane extending over the heads of the heads of the screws. Typically the membrane consists of a number of alternate layers of tar-impregnated felt paper and hot-mopped asphalt. If conventional screw fasteners are used in this assembly and large compressive forces, for example the weight of a person or a wheelbarrow, are applied to the top of the membrane between the screw heads, and the screw does not force its way downward through the threaded opening in the substrate, then the resilient insulation near the screw will be compressed, permitting the top membrane also to move downwardly adjacent the screw head, but the screw head will remain fixed and will tend to rupture the overlying membrane and pop upward through it, producing an undesirable source of water leakage through the membrane around the screw head.
In addition, if a conventional screw fastener is used and a large compressive force is applied downwardly on the screw head, the screw threads may be forced entirely through the threaded opening in the substrate and followed by a portion of the unthreaded shank of the screw. This can strip the threads in the opening in the substrate and/or increase the size of that opening to the size of the root of the threads, particularly where the screw threads are cut threads and the diameter of the shank is therefore as large as the root of the threads in the opening. If such an event occurs, the screw is thereafter easy to pull back up through the opening in the substrate, and the resistance to lift-off of the insulating layer and of any built-up roof on top of it is greatly reduced.
Thus when a conventional fasteners is used, compressive forces applied to the exterior of the covering can cause the insulation to be compressed and remain compressed, giving an uneven outer surface and reduced insulative properties; or can damage the substrate so that the lift-off resistance is decreased; or can cause the screw heads to pop up through the built-up roof and produce leaks. All of these are undesirable.
Accordingly, it is an object of the invention to provide a new and useful screw fastener, a new and useful assembly using a screw fastener to fasten a resilient layer, and a new and useful method of providing said assembly.
It is also an object to provide such a screw fastener, assembly and method of assembly which permits compression and reexpansion of a resilient layer held by the fastener, and mitigates the possibility of rupture of a covering overlying the head of the fastener when the resilient layer adjacent the fastener is compressed.